This study will compare giving d4T to giving the drug ZDV to children who are infected with HIV and who are aged 90 days to 6 years. In particular, the safety of giving d4T and how well it is tolerated will be studied in comparison to giving ZDV. It is hoped that d4T can be used to control HIV infection and slow the progression to AIDS. D4T is similar to AZT, but seems to have less bone marrow toxicity and no apparent other toxicity.